1. Field of the Invention
This invention relates to closed-loop compression refrigeration processes utilizing multi-stage compressors and a mixture of two or more refrigerants in the refrigeration process.
2. Description of the Related Art
In typical closed-loop compression refrigeration systems, refrigerant vapors are compressed and condensed by heat exchange. The condensate is expanded to a low pressure to produce a cooling effect which provides refrigeration duty. The refrigerant vapors from the expansion step are recycled to the compressor. The refrigerant in these systems can be a single component, such as ethylene, or a mixture of components such as propane and methane. Multi-component systems are generally used for lower temperature refrigeration.
There are several known processes utilizing multi-component refrigerants to achieve lower temperatures than that obtainable with a one component refrigerant. Examples include a cascade refrigeration system utilizing two or more separate compression loops, a multi-component refrigeration system similar to a single component system, or a separation process which partially condenses the compressed refrigerant and separates the vapor stream from the liquid stream to provide the cooler temperatures.
A cascade refrigeration system generally employs two or more compression loops wherein the expanded refrigerant from one stage is used to condense the compressed refrigerant in the next stage. Each successive stage employs a lighter, more volatile refrigerant which, when expanded, provides a lower level of refrigeration, i.e., is able to cool to a lower temperature. Such systems have the disadvantages of high cost because each stage of the cascade includes all of the components of a complete refrigeration system. Furthermore such systems have reduced reliability since the equipment in two or more complete compression loops are necessary to reach the desired refrigeration level.
Some multi-component refrigerant systems have no separation of a light and heavy phase. These systems operate in a manner very similar to a pure component system. While these systems are capable of obtaining colder temperatures than those achievable in pure component systems, they have several disadvantages. First, energy efficiency requires that the refrigerant composition be tailored to match the cooling curve of the process over the temperature range of interest. Second, the refrigeration system is much more difficult to operate because the composition of the refrigerant, which is usually three or four components, must be tightly controlled to be effective.
Other multi-component refrigeration systems separate a vapor and liquid stream in a primary separator after partial condensation. The purpose of this separation is to selectively route the condensed vapor stream to an expansion valve and heat exchanger to provide refrigeration at a cooler temperature.
For example, in U.S. Pat. No. 2,581,558, the multi-component refrigerant is compressed in a single stage compressor. This compressed refrigerant is partially condensed and the vapor stream, rich in the light component, is separated from the liquid stream in a primary separator. The liquid stream is split into two streams. The first stream is routed through an expansion valve and into a heat exchanger where it condenses the vapor from the primary separator and the second stream is routed through an expansion valve and into a heat exchanger where it cools an outside stream. The vapor from the primary separator, having been condensed in a heat exchanger, goes through an expansion valve and into another heat exchanger where it also cools an outside stream. The refrigerant vapors from the heat exchangers are combined, routed through several heat exchangers to provide heat integration, and returned to the suction of the compressor.
In U.S. Pat. No. 3,203,194, the multi-component refrigerant is compressed in a single stage compressor. This compressed refrigerant is partially condensed, and the vapor stream is separated from the liquid stream in a primary separator. The liquid stream is cooled in a heat exchanger, expanded across a valve and routed to a condenser where it condenses the vapors from the primary separator. The condensed primary separator vapor stream exiting from the condenser is expanded across a valve and routed through a heat exchanger to provide refrigeration duty. The mixed phase refrigerant from the exchanger is mixed with the liquid from the primary separator downstream of the expansion valve and upstream from the vapor condenser. After providing condensing duty, this combined stream is routed through two heat exchangers to provide heat integration, wherein the refrigerant is vaporized and returned to the suction of the compressor.
The refrigeration schemes shown in both U.S. Pat. Nos. 2,581,558 and 3,203,194 have several disadvantages. First, they are not optimally energy efficient because they do not obtain the maximum amount of refrigeration duty per compressor horse-power expended. These schemes do not optimize the driving force for heat exchange, which is the temperature differential between the two streams, nor do they compress the refrigerant in stages thereby reducing compressor horsepower. Second, these refrigeration systems do not achieve the lowest possible temperature upon expansion of the condensed primary separator vapor stream because these streams are not expanded to the lowest possible pressure, that of the compressor suction. The streams after the expansion valve and the heat exchanger providing the refrigeration duty must pass through several heat integration heat exchangers which cause pressure drops and which therefore increase the required pressure to which these streams must be expanded in order to ensure sufficient driving force to push the vapor through the heat exchangers to the compressor suction. Third, these refrigeration schemes are not flexible. They do not allow continuous, dynamic control of the temperature at the lowest level of refrigeration without significantly changing the pressure to which the refrigerant is expanded or significantly changing the pressure of the compressor discharge condenser and primary separator.
Thus, there is still a need in the industry for a multi-component refrigeration system which is more energy efficient, more flexible, and has improved operability over other multi-component refrigeration processes. This invention provides a high efficiency refrigeration system that achieves temperatures lower than comparable multi-component refrigeration systems while maintaining an ease of operation comparable to single component refrigeration systems.